Columbia Records in the early-60’s had an inner sleeve of very soft, very thin, limp plastic. It was a bag rounded on one side (following the curve of the LP), and perforated on the opposite. One had to rip open the bag liner along the perforation to unseal the LP.
I know this because my mother’s Ring Of Fire: The Best Of Johnny Cash LP was exactly so. I assume the outer cover was also shrink wrap sealed when it left the factory, so the OP’s LP is in one sense still sealed, the other way opened.